Formate dehydrogenase from Clostridiun pasteurianum catalyses the interconversion of CO 2 , and formate. It is a complex enzyme, containing molybdenum, iron, acid-labile sulfur and pterin. Redox potentiometry and EPR analysis reveal two spectrally and thermodynamically distinct iron-sulfur clusters, with g 1.87, 1.95 and 2.05 (E m 8 -318 mV) and g 1.92 and 2.05 (E m 8 -372 mV), both present at approx. 0.3 spins per enzyme. There is also a free radical signal centered near g 2.005 which we attribute to the pterin, perhaps in its anionic semiquinone form. This signal disappears on oxidation (E m 8 -150 mV), but is not reduced further at pH 8. It is present at approx. 0.8 spins per enzyme. No EPR signals attributable to the molybdenum were detected.